Steam generating unit



Juiy 2, 1963 E. c. MILLER 3,095,864

STEAM GENERATING UNIT Filed March 16, 1961 2 Sheets-Sheet 1 INVENTOR EARLE 0. MILLER BY M J '5 '4 /l 91',

ATTORNEY July 2, 1963 E. c. MILLER 3,09

STEAM GENERATING UNIT 2 Filed March 16, 1961 2 Sheets-Sheet 2 IIIIIIIIIII/IAVIIIIIIIIIIA VIIIIIIIIIIlI/IIIIWIIIIIIIIIIIII/II[mil/III WIIIIIIIIIIIIIIIIIIIIlIIIIIIIIIIIlI/flVII 1 INVENTOR EARLE 6. MILLER Mmw ORNEY 3,095,864 STEAM GENERATING UNIT Earle C. Miller, Worcester, Mass, assignor to Riley Stoker tlorporation, Worcester, Mass, a corporation of Massachusetts Fiietl Mar. 16, 1961, Ser. No. 96,241 Claims. (Q1. 12"2479) This invention relates to a steam generating unit and more particularly to apparatus arranged to regulate the temperature of superheated steam in a boiler.

In the generation of steam for use in producing electricity by means of a turbine, it is very necessary that the temperature of the steam reaching the turbine be maintained at a predetermined value with very little variation. Otherwise, the turbine will not operate at full efliciency. Furthermore, in present day steam generating units the temperature of the steam is just under the failure point of the metal in the superheater tubes and regulation of steam temperature is necessary to assure that the temperature of the steam does not go above the design value and cause tube failure. One of the methods commonly used for regulating the temperature of superheated steam is that of recirculation of gas to the main furnace from the back passes of the boiler. The effect of this recirculation is to cool the gases passing over the convection surfaces and also to increase the mass flow. Generally speaking, the increase in mass flow dominates the situation and the temperature goes up, despite the fact that the temperature of the gases passing over the superheater surface is lower. Because of the fact that the recirculated gas is used in considerably greater amounts and an increase in the heat transfer to the superheated steam is more necessary at low loads, a number of problems are presented. First of all, since the normal products of combustion are smaller in amount at low loads, they have difliculty mixing with the recirculated gas which has been introduced into the furnace. There are difliculties with ignition, for instance, and also in providing a suitable location for the introduction of the recirculated gas. These and other difficulties experienced with the prior art devices have been obviated in a novel manner by the present invention.

It is, therefore, an outstanding object of the invention to provide a novel apparatus for the control of superheat by the recirculation of gas.

Another object of this invention is the provision of a steam generating unit making use of the recirculation of gas from the back passes to the main furnace in which gas temperatures from side to side of the furnace are maintained at substantially uniform values.

It is another object of the present invention to provide a superheat control in which the recirculated gas is very thoroughly mixed with the ordinary products of combustion before passage through the boiler.

It is a further object of the invention to provide a steam generating unit embodying a novel method of introducing recirculated gas.

With these and other objects in view, as will be apparent to those skilled in the art, the invention resides in the combination of parts set forth in the specification and covered by the claims appended hereto.

The character of the invention, however, may be best understood by reference to one of its structural forms, as illustrated by the accompanying drawings in which:

FIG. 1 is a vertical longitudinal view of a steam generating unit embodying the principles of the present invention;

FIG. 2 is a sectional view of a burner of the apparatus taken along the line IIII of FIG. 3; and

rates rem ire FIG. 3 is a sectional view of the apparatus taken along the line III-III of FIG. 1.

Referring first to FIG. 1, wherein is best shown the general features of the invention, the steam generating unit, indicated generally by the reference numeral 10, is shown as consisting of a furnace 11, and a boiler 12. The steam generating unit is mounted on a supporting structure '13. The furnace '11 is provided with a front wall 14, a rear Wall 15, and side walls 16 defining a vertically elongated combustion chamber 17. The furnace is of the type general-1y shown and described in the patent of Craig No. 2,853,059 and is provided with a continuous slag bottom 18. The front wall 14 is provided with an abutment 19 which extends inwardly of the furnace; the abutment has an inclined upper surface 21 and a similarly inclined lower surface 22. The rear wall 15 is provided with a similar abutment 23 having an inclined upper surface 24 and an inclined lower surface 25. The inclined lower surfaces of the two abutments are provided with directional-flame intertube burners 26 and 27 of the type shown in the patent to Craig No. 2,759,460 which are connected in the usual way with sources of air and fuel. The abutments 19 and 23 extend in opposition to each other across the combustion chamber and divide the combustion chamber into a lower hightemperature cell 28 and an upper portion 29. The boiler 12, which cooperates with the furnace 11 in the usual way, consists of a steam-and-water drum 31 from which radiate the various tubes of the boiler system. Large downcomer tubes 30 carry the water to the lower portion of the boiler and from there it flows upwardly through the system, particularly in water-wall tubes 20 which lie on the surface of the combustion chamber 17 and particularly cover the surfaces of the abutments 19 and 23. Directional-flame burner-s .26 and 27 are so arranged that the fuel and combustion air passes through gaps provided between certain of the water-wall tubes. Steam generated in the waterwall tubes 20 is introduced into the steam-and-water drum 31. At the upper part of the drum are tubes 32 leading to a primary or low-temperature superheater 33. The outlet end of the low-temperature superheater 33 is connected to a secondary or high temperature superheater 34, the out-let of which is connected to the turbine (not shown) in the usual manner.

The steam generating unit is provided with a back pass 35 divided by a wall 36 into a forward pass 37 and a rearward pass 38. The low-temperature superheater 33 is located in the rearward pass 38 and a low-temperature reheater 39 is located in the forward pass 37. At the upper portion of the combustion chamber 17 the rearward wall 15 is inclined forwardly to form a nose 41 which defines a short horizontal upper pass 42. A hightemperature reheater 43 resides in this upper pass as does the terminal portion of the high-temperature superheater 34. The major portion of the high-temperature superheater 34, however, resides in the upper portion of the combustion chamber 17 and consists of large pendant platens. The lower ends of the platens of the high-temperature superheater 34 extend below the nose 41 into the upper part of the combustion chamber.

In the lower portion of the rearward pass 38 are located dampers 44 which control the flow of gas therethrough. These dampers are connected to an actuating rod 45 so as to operate together and the rod 45 is connected to a linear actuator, such as a hydraulic cylinder 46. Hydraulic lines 47 and 48 lead to a main control 49 and are connected to opposite ends of the cylinder 46. The said main control 49 is similar to that shown and described in the copending patent application of Parmakian Serial Number 774,074, filed November 14, 1958, assigned to the same assignee as the instant application. In the lower end ofthe forward pass 37 are located -ner. duct 92 is located a slide valve 97 operated by a cylinder dampers 51 which are'movable together into the action of a rod 52 connected to a hydraulic cylinder 53 which operates under the control of hydraulic lines 54 and 55 connected to the main control 49. The lower end of the back pass 35 of the steam generating unit is connected through an air heater 56 to a breaching 57 which leads to a stack. The'outlet of the low-temperature reheater 33 is connected to a reheated steam header 58, while the outlet of the high-temperature superheater 34 is connected to a superheated steam header 59.

The lower surface of the abutment 19 is provided with a series of the burners 26, while a similar series of burners 27 is provided on the under surface of the abutment 23, as has been described above. A large conduit 61 extends parallel to the front wall 14 outside of the burners 26, While a similar conduit 62 extends outside of the burners 27 and parallel to the rear wall 15 of the furnace. These conduits are joined by a duct 60 and are also joined to alarger single conduit 65 having mounted therein a regulating damper 66. The end of the conduit 65 which is away from the connection to the conduits 61 and 62 is connected to the outlet of a fan 67. The inlet of the fan 67 is connected by a conduit 68 to a portion of the back passes, such as the portion of the steam generating unit lying between the dampers 54 and 55 on the one hand and the air heater 56 on the'other hand.

The damper 66 ispivotally mounted and is actuated --through rods 69 and 71, the latter rod being connected to a hydraulic cylinder 72 whose linear position is determined by the fiuid pressure hydraulic lines 73 and 74 connected to the main control 49. The main control is connected through a line 75 to a temperature-indicating 'device such as a thermocouple 76 located in the superheated steam header 59. The main control is connected in a similar manner by line 77 to a temperature-indicating device such as a thermocouple 78 lying in the reheated steam header 58.

FIG. Zshows the manner in which the'burners 26 are constructed. The burner openings are provided by bringing back certain of the water-wall tubes 20. A main fuel gun 81 extends centrally of the burner and is connected to a conduit 82 from which it receives the powdered coal and primary air. A gas gun 83 is located centrally of the main fuel gun maintaining ignition and an igniter 84 is located in the burner closely adjacent the main fuel gun, the igniter being of the electric spark variety and using gas as its fuel. Arranged in the burner above the gun 81 are vanes 85 whose inclination to the horizontal is adjustable by means of a control rod 86. In the lower part of the burner, vanes 87 are controlled by a rod 88. Air arrives at the burner through a duct 89.

FIG. 3 shows the manner in which the recirculated gas ducts and the burners are related. The drawing shows the recirculated ducts61 extending parallel to the front wall 14 of the furnace outside of the air plenum chambers of the burners 26. This View particularly shows the manner in which the water-Wall tubes 20 are bent back to form the spaces in the burners for the fuel guns '81 and the vanes 85 and 87. As is evident in FIG. 1,

the air from the air heater 56 arrives at the burner 26 through a duct 91. The duct 91 is connected to each burner by its duct 89. Extending downwardly from the recirculated gas conduit 61 is a short duct section 92 having a slide valve 93 operated by an actuator 94. The actuator'is connected by lines 95 and 96 to the main cont ol 49. It should be noted that only alternate burners 26 are connected to the gas duct 61 in this man- In the duct 89 below the connection through the 98 and is connected by lines 99 and 101 to the main con- :trol 49. It will be observed in FIG. 1 that each of the burners 27 associated with the abutment 23 is provided 104 is in parallel with the cylinder 98 and is operated by the same impulses. Similarly, the recirculated gas duct 62 is connected by a short duct to the fuel-air duct 102 above the valve 103 and the duct 105- is provided with a slide valve 106 operated by a cylinder 107. This cylinder is connected to the lines 95 and 96 coming from the main control 49 and, therefore, the cylinder 107 is in parallel With the cylinder 94 and is operated by the same impulse. The pipe 82 which furnishes the burner 26 with fuel joins with a similar pipe 108 connected to the burners 27 and the two join for connection to a main pipe 109 leading to the source of fuel, such as a pulverizer (not shown). In the pipe 109 is located a valve 111 for controlling the amount of fuel flowing to the burners, 'and this valve is connected for actuation to a cylinder 112, which, in turn, is connected by lines 113 and 114 to the main control 49.

The upper or exit end of the low-temperature superheater 33 is connected by a pipe 115 to the inlet end of the high-temperaiture superheater 34. Located in this pipe is a steam desuperheater 116 to which is connected a water pipe 117. In this pipe is connected a valve 118 which is actuated to various positions by a hydraulic cylinder 119. The cylinder is connected by lines 121 and 122 to the main control 49. Located in the superheated steam header 59 is an orifice 123 and a line 124 is connected to the header 59 of one side of this orifice and a line 125 is connected to the other side. These two lines are connected to the master control 49 to give a signal indicative of the amount of steam generated, or in other words, of load. A signal indicative of load could also have been obtained by measurements taken in a venturi located in the air duct 91.

The operation of the invention will now be readily understood in View of the above description. Fuel and air leave the burners 26 and 27 and enter the high-temperature cell 28 of the combustion chamber 17. The cell 28 exists at a high temperature because of the restricted exit between the abutments 19 and 23. Extreme turbulence takes place because of the opposed condition of the burners and because of the presence in the cell of a pool of molten slag which accumulates in the bottom of the furnace. A considerable portion of the combustion takes place in the cell 28 and because of the high temperature, the formation of slag takes place almost completely in this high-temperature cell and is persuaded to remain there. The products of combustion pass upwardly through the combustion chamber 17 and a certain amount of combustion takes place in the upper portion 29. The products of combustion leave the upper portion of the combustion chamber, pass through the horizontal pass 42 and then pass downwardly through the back pass 45. The amounts passing through the forward portion 37 and the rearward portion 38 of theback pass are determined by the settings of the 'dampers'44 and 5-1. Gases pass through the air heater 56 passes upwardly through the low-temperature superheater 33 in counterflow to the flow of gases. After being superheated in this manner to a certain degree, the steam then passes into the secondary or high-temperature superheater 34 entering the portion thereof adjacent the front wall 14. The passage of steam between the two sections of the superheater takes place through the pipe 115 The steam in the high-temperature superheater is first subjected to considerable amount of radiation because of the fact that most of this portion of the superheater lies in the platens,

which extend deeply into the main combustion chamber 17. The'steam is then subjected to a small degree of convection superheating, passes into the superheated steam header 59, and flows from there to the turbine.

After passing through the high-pressure section of the turbine, the steam returns to the boiler for reheating. It enters the primary reheat section 39 in the lower portion and passes upwardly therethrough in counter flow to the flow of gases in the forward portion 37 of the back pass 35. Eventually, the steam reaches the high-temperature section 43 of the reheater, passes forwardly into the horizontal pass 42 in counterflow to the fiow of gases therethrough and eventually enters the reheated steam header 58 for passage to the low-pressure section of the turbine. The superheat and reheat temperatures can be controlled to a certain extent in the usual way by regulating a portion of the total amount of products of combustion which pass through the forward portion 37 or the rearward portion 38 of the back pass. Funthermore, the final superheat temperature reaching the superheated steam header 59 is regulated by the main control through the lines 121 and 122 operating the cylinder 119 which actuates the valve 118. This controls the amount of water flowing from the pipe 117 into the desuperheater 116 of the pipe 115 and this regulates the final steam temperature reaching the turbine. It should be understood in connection with the dampers 44 that the main control operates through the lines 47 and 48 to actuate the cylinder 46 to regulate these dampers. In a similar manner the dampers 51 are manipulated by the main control operating through the lines 54 and 55 to actuate the cylinder 53 and Work through the rod 52 to adjust the dampers. This damper control not only may be used for adjusting the ratio between reheat steam temperature and superheat steam temperature, but also to adjust the actual value of both of them by a bypass arrangement. This by-pass arrangement is not shown in detail but is operated by dampers 126 similar to the dampers 34 and 51 through an actuator 127 connected to the main control. Although this by-pass is not shown in any of the figures of the drawings, it will be understood that it would be located centrally of the back-pass 35 midway between the two sides of the steam generating unit. It is not possible, however, to accomplish complete control of the steam temperature values by means of the desuperheater and the damper control alone. By properly setting the damper 66, a certain amount of the products of combustion can be carried back into the furnace. This damper, of course, is actuated by an impulse from the main control 49 passing through the lines 73 and 74 to the cylinder 72, which operates through the bars 71 and 69 to set the damper and control the type of flow of gas recirculated back to the furnace. These products leave the back pass of the furnace through the conduit 68 under the impetus of the fan 67 and enter the conduit 65. They are then divided between the conduits 61 and 62 and they may pass downwardly through the ducts 92 and 105, respectively. If the valve 93 is open, the gas is permitted to pass into the duct 89 leading to the burner 26. This is particularly true when the valve 97 is closed so that no air passes upwardly to the duct 89. The position of the valves 93 and 98 are dictated by the main control 49 by means of the signal lines and cylinders. A similar situation is true in connection with the burners 27. As has been stated above, only alternate burners are connected by ducts to the gas recirculation ducts 61 and 62. The introduction of large amounts of gas which have been recirculated in this manner has the effect of increasing the mass flow of gases over the heat exchange unit in the boiler. With respect to the convection sections of a heat exchanger, an increase in mass flow (even though this increase is brought about by the introduction of lower temperature gases) results in a greater amount of heat transfer and a raising of the temperature of steam flowing within the heat exchanger. Ordinarily. of course, the recirculation of gas is reserved only for those portions of load in which substantial increases in unaided superheat and reheat are necessary. The contemplation of the present invention is that the burners 26 and 27 which are connected to the recirculated gas ducts would be used either as burners or as recirculated gas openings but never as both. When a suitable low load is reached, as indicated to the main control by the change in the pressure drop through the orifice 12/3 in the reheated steam header 59, this signal being indicated through the lines 124 and 125, the main control will send a signal out which will close the air slide valves 97 and 103 and open the valves 93 and 106. This means, of course, that the burners which are not connected to the recirculated gas will receive more fuel and air than they would if the load were distributed among all of the burners. At low loads in most furnaces the burners operate at a lower rating but, according to the present invention, the burners would be operated at somewhat the same high rating as they would at full load. The effect of this is to promote better ignition and so on in the burners. At the same time, it is contemplated that at low loads the vanes in the burners would all be raised when this recirculated gas goes through them, so that the recirculated gas flows immediately upwardly over the noses 19 and 23, whereas the other burners remain with their vanes and 87 in a lowered position, tilting somewhat toward the center of the high-temperature cell 28. This alternate arrangement of the gas recirculating vanes and the burner vanes will give a very beneficial result in that the recirculated gas will immediately go up over the noses 19 and 23 and be more or less segregated from the products of combustion which, after swirling through the center of the high-temperature cell 28 in the usual manner, will rise through the center of the furnace. This will tend to protect the front wall 14 and the rear wall 15 from slagging.

Certain minor changes may be made in the form and construction of the invention Without departing from the material spirit thereof. It is not, however, desired to confine the invention to the exact form herein shown and described, but it is desired to include all such as properly come within the scope claimed.

The invention having been thus described, what is new and desired to secure by Letters Patent is:

1. Apparatus for controlling superheat in a steam generating unit, comprising Walls defining a furnace, a back pass through which the products of combustion travel after leaving the furnace, an abutment on one Wall forming a restriction in the furnace, a cell containing burner openings located on the lower surface of the abutments, a first conduit connected at one end to certain of the burner openings, a fan whose outlet is connected to the other end of the conduit, a second conduit connecting the inlet of the fan to the said back pass, fuel and air mixture means connected to the burner openings, a valve located in the first conduit for regulating gas flow therethrough, a valve associated with the fuel and air mixture means for regulating the flow thereof through the burner openings, a convection superheater located downstream of the said openings, an indicator associated with the said convection superheater producing a signal proportional to the degree of superheat, actuators for determining the setting of the valves, control means receiving the signal from the indicator and transmitting a signal to the actuator to maintain the degree of superheat at a selected predetermined value, the valves opening and closing to permit flow of either gas alone or mixture alone through the certain of the burner openings to maintain the temperature of superheat at a predetermined value.

2. Apparatus for controlling superheat in a steam generating unit, comprising walls defining a vertically-elongated furnace, a back pass through which the products of combustion travel after they leave the furnace, abutrnents on opposed walls to form a restriction in the furnace, a slagging cell located below the abutments, a series of burner openings into the furnace located on the lower surfaces of the abutments, a combustion air duct connected to the burner openings, at first conduit connected at one end to certain of the burner openings, a fan whose outlet is connected to the other end of the conduit, a second conduit connecting the inlet of the fan to the said back pass, a valve located in the first conduit for regulating gas assasea flow therethrough, a valve located in the combustion air duct for regulating air flow therethrough, a convection superheater located downstream of the said opening, an indicator associated with the superheater producing a signal proportional to the degree of superheat, apparatus for determining the setting of the valve, control means receiving the signal from the indicator and transmitting signals to the actuator to maintain a degree of superheat at a selected predetermined value, the valves opening and closing to permit flow of either gas alone or combustion air alone to the certain of the burner openings to main tain the temperature of superheat at a predetermined value.

3. Apparatus for controlling superheat in a steam generating unit, comprising walls defining a vertically-elongated furnace, a back pass through which the products of combustion travel after they leave the furnace, abutments on opposed walls, each abutment having inclined upper and lower surfaces intersecting inwardly of the furnace walls to form a restriction in the furnace, the furnace being provided with a slagging bottom, directional flame burners located'on the said lower inclined surfaces of the abutments, a fuel pipe connected to the burners, a first conduit connected at one end to certain of the burners, a fan whose outlet is connected to the other end of the conduit, a second conduit connecting the inlet of the fan to the said back pass, a valve for stopping the gas flow to the openings,-a valve in the fuel pipe for stopping fuel flow to the burners, a convection superheater located downstream of the said burners, an indicator associated with the superheater producing a signal representative of the degree of superheat, actuators for determining the setting of the said valves, and control means receiving the signal from the indicator and transmitting signals to the actuator to maintain the degree of superheat at a selected predetermined value, the actuators closing one valve and opening the other to allow orto prevent the flow of gas alone through the first conduit to the burners while at the same time alternately preventing or allowing the flow of fuel only to the certain of the bumers to maintain the temperature of superheat at a predetermined value.

4. Apparatus for controlling superhea-t in a steam generating unit, comprising walls defining a verticallyelongated furnace, a back pass through which the products of combustion travel after they leave the furnace, the said back pass having a convection superheater unit and a convection reheater unit in parallel located downstream of the furnace, abutments on opposed walls of the furnace, each abutment having inclined upper and lower surfaces intersecting inwardly of the furnace walls to form a restriction in the furnace, the furnace being provided with a slagging bottom, directional-flame burners located on the said lower inclined surfaces of the abutments, dampers controlling the division of the flow of gas from the back pass over the superheater and reheater units, a first conduit connected at one end to the certain of the burners, a fan whose outlet is connected to the other end of the conduit, a second conduit connecting the inlet of the fan to the said back pass, a valve in the first conduit for regulating the gas flow to the certain of the burners, a valve associated with a combustion mixture supply, indicators associated with the superheater and reheater elements producing signals representative of the degree of superheat and reheat, actuator for determining the set-ting of the said valves, an actuator for determining the set-ting of the said dampers, and control means receiving the signals from the indicator and transmitting signals to the actuator to open one valve and close the other valve to permit flow of either gas alone or combustion mixture alone to the said certain of the burners to maintain the degree of superheat land reheat at selected predetermined values. 7

5. Apparatus for controlling superheat and reheat in a steam generating unit, comprising walls defining a vertically-elongated furnace, a back pass through which the products of combustion travel after they leave the furnace, a superheater located downstream of the furnace, a reheater located downstream of the furnace, differential means for controlling the difference between the superheat and reheat temperatures, abutments on opposed walls of the furnace, each abutment having inclined upper and lower surfaces intersecting inwardly of the furnace walls to form a restriction in the furnace, the furnace being provided with a slugging bottom, directional-flame burners located on the said lower inclined surfaces of the abutments, a preheated air duct connected to at least one of the burners, a first conduit connected at one end to the said one of the burners, a fan whose outlet is connected to the other end of the conduit, a second conduit connecting the inlet of the fan to the said back pass, a valve located in the first conduit for shutting off gas flow \therethrough, a valve located in the preheated air duct to shut oif the flow of air therethrough, indicators associated with the superheater and reheater producing signals proportional to the degree of superheat and reheat, actuator for determining the settings of the valve, and control means receiving signals in the indicator and transmitting signals to the actuator to shut off one valve and open the other and to the difi'erential means to maintain the degree of superheat and reheat at selected predetermined value.

References Cited in the file of this patent UNITED STATES PATENTS Great Britain July 24, 1940 

1. APPARATUS FOR CONTROLLING SUPERHEAT IN A STEAM GENERATING UNIT, COMPRISING WALLS DEFINING A FURNACE, A BACK PASS THROUGH WHICH THE PRODUCTS OF COMBUSTION TRAVEL AFTER LEAVING THE FURNACE, AN ABUTMENT ON ONE WALL FORMING A RESTRICTION IN THE FURNACE, A CELL CONTAINING BURNER OPENINGS LOCATED ON THE LOWER SURFACE OF THE ABUTMENTS, A FIRST CONDUIT CONNECTED AT ONE END TO CERTAIN OF THE BURNER OPENINGS, A FAN WHOSE OUTLET IS CONNECTED TO THE OTHER END OF THE CONDUIT, A SECOND CONDUIT CONNECTING THE INLET OF THE FAN TO THE SAID BACK PASS, FUEL AND AIR MIXTURE MEANS CONNECTED TO THE BURNER OPENINGS, A VALVE LOCATED IN THE FIRST CONDUIT FOR REGULATING GAS FLOW THERETHROUGH, A VALVE ASSOCIATED WITH THE FUEL AND AIR MIXTURE MEANS FOR REGULATING THE FLOW THEREOF THROUGH THE BURNER OPENINGS, A CONVECTION SUPERHEATER LOCATED DOWNSTREAM OF THE SAID OPENINGS, AN INDICATOR ASSOCIATED WITH THE SAID CONVECTION SUPERHEATER PRODUCING A SIGNAL PROPORTIONAL TO THE DEGREE OF SUPERHEAT, ACTUATORS FOR DETERMINING THE SETTING OF THE VALVES, CONTROL MEANS RECEIVING THE SIGNAL FROM THE INDICATOR AND TRANSMITTING A SIGNAL TO THE ACTUATOR TO MAINTAIN THE DEGREE OF SUPEREHEAT AT A SELECTED PREDETERMINED VALUE, THE VALVES OPENING AND CLOSING TO PERMIT FLOW OF EITHER GAS ALONE OR MIXTURE ALONE THROUGH THE CERTAIN OF THE BURNER OPENINGS TO MAINTAIN THE TEMPERATURE OF SUPERHEAT AT A PREDETERMINED VALUE. 